Pure ethylene is photo-polymerized with an exciplex laser on the KrF line (248 nm) at temperatures from 190 °C to 230 °C and for pressures up to 3 200 bar. The radical reaction is directly monitored via quantitative high-pressure high-temperature near infrared spectroscopy. Toward higher densities and temperatures, the overall quantum yield increases and up to several thousand ethylene molecules are polymerized by one absorbed laser photon. A kinetic scheme is presented which adequately describes the polymerization kinetics as a function of temperature, overall density, laser pulse energy, and conversion. Termination rate coefficient kt is found to decrease in the initial polymerization period, whereas a single value of kt is sufficient to represent kinetic data in the extended conversion range from 20% to 70%.